MRI has achieved prominence as a diagnostic imaging modality, and increasingly as an interventional imaging modality. The primary benefits of MRI over other imaging modalities, such as X-ray, include superior soft tissue imaging and avoiding patient exposure to ionizing radiation produced by X-rays. MRI's superior soft tissue imaging capabilities have offered great clinical benefit with respect to diagnostic imaging. Similarly, interventional procedures, which have traditionally used X-ray imaging for guidance, stand to benefit greatly from MRI's soft tissue imaging capabilities. In addition, the significant patient exposure to ionizing radiation associated with traditional X-ray guided interventional procedures is eliminated with MRI guidance.
MRI uses three fields to image patient anatomy: a large static magnetic field, a time-varying magnetic gradient field, and a radiofrequency (RF) electromagnetic field. The static magnetic field and time-varying magnetic gradient field work in concert to establish proton alignment with the static magnetic field and also spatially dependent proton spin frequencies (resonant frequencies) within the patient. The RF field, applied at the resonance frequencies, disturbs the initial alignment, such that when the protons relax back to their initial alignment, the RF emitted from the relaxation event may be detected and processed to create an image.
Each of the three fields associated with MRI presents safety risks to patients when a medical device is in close proximity to or in contact either externally or internally with patient tissue. One important safety risk is the heating that can result from an interaction between the RF field of the MRI scanner and the medical device (RF-induced heating), especially medical devices which have elongated conductive structures with tissue contacting electrodes, such as electrode wires in pacemaker and implantable cardioverter defibrillator (ICD) leads, guidewires, and catheters. Thus, as more patients are fitted with implantable medical devices, and as use of MRI diagnostic imaging continues to be prevalent and grow, the need for safe devices in the MRI environment increases.
A variety of MRI techniques are being developed as alternatives to X-ray imaging for guiding interventional procedures. For example, as a medical device is advanced through the patient's body during an interventional procedure, its progress may be tracked so that the device can be delivered properly to a target site. Once delivered to the target site, the device and patient tissue can be monitored to improve therapy delivery. Thus, tracking the position of medical devices is useful in interventional procedures. Exemplary interventional procedures include, for example, cardiac electrophysiology procedures including diagnostic procedures for diagnosing arrhythmias and ablation procedures such as atrial fibrillation ablation, ventricular tachycardia ablation, atrial flutter ablation, Wolfe Parkinson White Syndrome ablation, AV node ablation, SVT ablations and the like. Tracking the position of medical devices using MRI is also useful in oncological procedures such as breast, liver and prostate tumor ablations; and urological procedures such as uterine fibroid and enlarged prostate ablations.
The RF-induced heating safety risk associated with electrode wires in the MRI environment results from a coupling between the RF field and the electrode wire. In this case several heating related conditions exist. One condition exists because the electrode wire electrically contacts tissue through the electrode. RF currents induced in the electrode wire may be delivered through the electrode into the tissue, resulting in a high current density in the tissue and associated Joule or Ohmic tissue heating. Also, RF induced currents in the electrode wire may result in increased local specific absorption of RF energy in nearby tissue, thus increasing the tissue's temperature. The foregoing phenomenon is referred to as dielectric heating. Dielectric heating may occur even if the electrode wire does not electrically contact tissue, such as if the electrode was insulated from tissue or if no electrode was present. In addition, RF induced currents in the electrode wire may cause Ohmic heating in the electrode wire, itself, and the resultant heat may transfer to the patient. In such cases, it is important to attempt to both reduce the RF induced current present in the electrode wire and to limit the current delivered into the surrounding tissue.
Methods and devices for attempting to solve the foregoing problem are known. For example, high impedance electrode wires limit the flow of current and reduce RF induced current; a resonant LC filter placed at the wire/electrode interface may reduce the current delivered into the body through the electrodes, non-resonant components placed at the wire/electrode interface may also reduce the current transmitted into the body; and co-radial electrodes wires may be used to provide a distributed reactance along the length of the wire thus increasing the impedance of the wire and reducing the amount of induced current.
Notwithstanding the foregoing attempts to reduce RF-induced heating, significant issues remain. For example, high impedance electrode wires limit the functionality of the electrode wire and do not allow for effective ablation, pacing or sensing. Resonant LC filters placed at the wire/electrode interface inherently result in large current intensities within the resonant components resulting in heating of the filter itself, at times exceeding 200.degree. C. Additionally, a resonant LC filter at the wire/electrode interface can result in a strong reflection of the current induced on the electrode wire and may result in a standing wave that increases the temperature rise of the wire itself and/or results in increased dielectric heating near the electrode wire which in turn heats surrounding tissue to potentially unacceptable levels and may melt the catheter or lead body in which it is housed. Non-resonant components alone do not provide sufficient attenuation to reduce the induced current to safe levels. Additionally, the components will experience a temperature rise, if the conductor cross-sectional area is too small. While an electrode wire with distributed reactance (i.e. coiled wires) can reduce the level of induced current on the wire, it does not sufficiently block the current that is induced on the wire from exiting the wire through the electrodes. Thus, while coiled wires may work for certain short lengths or distances, in situations requiring longer lengths or distances, coiled wires do not by themselves provide enough impedance to block current.
Current technologies for reducing RE-induced heating in medical devices, especially those with elongated conductive structures such as electrode wires, are inadequate. Therefore, new electrode wire constructs and lead or catheter assemblies are necessary to overcome the problems of insufficient attenuation of RF energy.